Systems and Methods for Controlling Rotation and Twist of a Tether

ABSTRACT

A system may include a tether, a slip ring, a tether gimbal assembly, a drive mechanism, a control system. The tether may include a distal tether end coupled to an aerial vehicle, a proximate tether end, and at least one insulated electrical conductor coupled to the aerial vehicle. The slip ring may include a fixed portion and a rotatable portion, where the rotatable portion is coupled to the tether. The tether gimbal assembly may be rotatable about at least one axis and is coupled to the fixed portion of the slip ring. The drive mechanism may be coupled to the slip ring and configured to rotate the rotatable portion of the slip ring. And the control system may be configured to operate the drive mechanism to control twist in the tether.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/586,909, filed Dec. 30, 2014, which claims priority to U.S.Provisional Application No. 62/019,273, filed Jun. 30, 2014. The entiredisclosure contents of U.S. application Ser. No. 14/586,909 and U.S.Provisional Application No. 62/019,273 are herewith incorporated byreference into the present application.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Power generation systems may convert chemical and/or mechanical energy(e.g., kinetic energy) to electrical energy for various applications,such as utility systems. As one example, a wind energy system mayconvert kinetic wind energy to electrical energy.

SUMMARY

Systems and methods for controlling rotation and twist of a tether aredescribed herein. More specifically, example embodiments generallyrelate to systems that incorporate a ground station for tethering aerialvehicles. During certain flight modes, the tether connecting the aerialvehicle to the ground station may twist as the aerial vehicle orbitsabout an axis relative to the ground station. Beneficially, embodimentsdescribed herein may control rotation and twist of the tether so as toavoid breaking components of the tether and/or improve a fatigue life ofthe tether.

In one aspect, an example system may include a tether that includes adistal tether end coupled to an aerial vehicle, a proximate tether end,and at least one insulated electrical conductor coupled to the aerialvehicle; a slip ring that includes a fixed portion and a rotatableportion, where the rotatable portion is coupled to the tether; a tethergimbal assembly, where the tether gimbal assembly is rotatable about atleast one axis and is coupled to the fixed portion of the slip ring; adrive mechanism coupled to the slip ring and configured to rotate therotatable portion of the slip ring relative to the fixed portion; and acontrol system configured to operate the drive mechanism to controltwist in the tether.

In another aspect, a system may include a ground station; a tether thatincludes a distal tether end coupled to an aerial vehicle, a proximatetether end, and at least one insulated electrical conductor coupled tothe aerial vehicle, a slip ring that includes a fixed portion and arotatable portion, where the fixed portion is coupled to the groundstation and the rotatable portion is coupled to the tether; a tethergimbal assembly, where the tether gimbal assembly is rotatable about atleast one axis, and where the tether passes through the tether gimbalassembly; a drive mechanism coupled to the slip ring and configured torotate the rotatable portion of the slip ring relative to the fixedportion; and a control system configured to operate the drive mechanismto control twist in the tether.

In another aspect, a system may include a tether that includes a distaltether end coupled to an aerial vehicle; a proximate tether end; and atleast one insulated electrical conductor coupled to the aerial vehicle;a slip ring comprising a fixed portion and a rotatable portion, wherethe rotatable portion is coupled to the tether; a tether gimbalassembly, where the tether gimbal assembly is rotatable about at leastone axis; and a resistive bearing system coupled to the slip ring, wherethe resistive bearing system is configured to allow the rotatableportion of the slip ring to rotate relative to the fixed portion when atorque provided by the tether exceeds a slip limit, and furtherconfigured to inhibit the rotation of the rotatable portion of the slipring relative to the fixed portion when the torque provided by thetether does not exceed the slip limit.

In another aspect, a system may include a tether that includes a distaltether end coupled to an aerial vehicle; a proximate tether end; and atleast one insulated electrical conductor coupled to the aerial vehicle;a slip ring comprising a fixed portion and a rotatable portion, wherethe rotatable portion is coupled to the tether; a tether gimbalassembly, where the tether gimbal assembly is rotatable about at leastone axis; and a resistive bearing system coupled to the slip ring, wherethe resistive bearing system is configured to allow the rotatableportion of the slip ring to rotate relative to the fixed portion and toprovide a resistance to the rotational torque of the tether so as tomaintain the twist in the tether within a determined range of values.

In another aspect, a method may involve launching an aerial vehicleconnected to a tether, transitioning the aerial vehicle to crosswindflight, and controlling, by a control system, an amount of twist in thetether during crosswind flight.

In yet another aspect, a system may include means for launching anaerial vehicle connected to a tether, means for transitioning the aerialvehicle to crosswind flight, and means for controlling an amount oftwist in the tether during crosswind flight.

In another aspect, a system may include a tether that includes a distaltether end coupled to an aerial vehicle; a proximate tether end; and atleast one insulated electrical conductor coupled to the aerial vehicle;a tether gimbal assembly, where the tether gimbal assembly is coupled tothe tether and is rotatable about at least one axis; a drive mechanismcoupled to the tether and configured to rotate the tether; and a controlsystem configured to operate the drive mechanism to control twist in thetether.

In another aspect, a system may include a tether that includes a distaltether end coupled to an aerial vehicle; a proximate tether end; and atleast one insulated electrical conductor coupled to the aerial vehicle;a slip ring comprising a fixed portion and a rotatable portion, whereinthe rotatable portion is coupled to the tether; a tether gimbalassembly, wherein the tether gimbal assembly is rotatable about at leastone axis and is coupled to the fixed portion of the slip ring; a drivemechanism coupled to the slip ring and configured to rotate therotatable portion of the slip ring relative to the fixed portion; and acontrol system configured to operate the drive mechanism to controltwist in the tether.

In another aspect, a system may include a tether that includes a distaltether end coupled to an aerial vehicle; a proximate tether end; and atleast one insulated electrical conductor coupled to the aerial vehicle;a tether gimbal assembly, where the tether gimbal assembly is coupled tothe tether and is rotatable about at least one axis; and a resistivebearing system coupled to the tether gimbal assembly, where theresistive bearing system is configured to allow the proximate tether endto rotate when a torque at the proximate tether end exceeds a sliplimit, and further configured to inhibit the rotation of the proximatetether end when the torque does not exceed the slip limit.

In yet another aspect, a system may include a tether that includes adistal tether end coupled to an aerial vehicle; a proximate tether end;and at least one insulated electrical conductor coupled to the aerialvehicle; a tether gimbal assembly, where the tether gimbal assembly iscoupled to the tether and is rotatable about at least one axis; and aresistive bearing system coupled to the tether gimbal assembly, wherethe resistive bearing system is configured to allow the proximate tetherend to rotate and to provide a resistance to the rotational torque ofthe tether so as to maintain the twist in the tether within a determinedrange of values.

These as well as other aspects, advantages, and alternatives, willbecome apparent to those of ordinary skill in the art by reading thefollowing detailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts an Airborne Wind Turbine (AWT), according to an exampleembodiment.

FIG. 2 is a simplified block diagram illustrating components of an AWT,according to an example embodiment.

FIG. 3 depicts an aerial vehicle, according to an example embodiment.

FIGS. 4a-c illustrate twist in a tether, according to an exampleembodiment.

FIG. 5a depicts an aerial vehicle coupled to a ground station via atether, according to an example embodiment.

FIG. 5b depicts an aerial vehicle coupled to a ground station via atether, according to an example embodiment.

FIG. 6a depicts a system for controlling rotation and twist of a tether,according to an example embodiment.

FIG. 6b depicts a foreshortened view of a tether section, according toan example embodiment.

FIG. 7 depicts a system for controlling rotation and twist of a tether,according to an example embodiment.

FIG. 8 depicts a tether in cross-section, according to an exampleembodiment.

FIG. 9 depicts a tether in cross-section, according to an exampleembodiment.

FIG. 10 depicts a tether in cross-section, according to an exampleembodiment.

FIG. 11 is a flow chart illustrating a method, according to an exampleembodiment.

DETAILED DESCRIPTION

Exemplary systems and methods are described herein. It should beunderstood that the word “exemplary” is used herein to mean “serving asan example, instance, or illustration.” Any embodiment or featuredescribed herein as “exemplary” or “illustrative” is not necessarily tobe construed as preferred or advantageous over other embodiments orfeatures. More generally, the embodiments described herein are not meantto be limiting. It will be readily understood that certain aspects ofthe disclosed systems and methods can be arranged and combined in a widevariety of different configurations, all of which are contemplatedherein.

I. OVERVIEW

Illustrative embodiments relate to aerial vehicles, which may be used ina wind energy system, such as an Airborne Wind Turbine (AWT). Inparticular, illustrative embodiments may relate to or take the form ofsystems for controlling rotation and twist of a tether that connects anaerial vehicle to a ground station.

By way of background, an AWT may include an aerial vehicle that flies ina closed path, such as a substantially circular path, to convert kineticwind energy to electrical energy. In an illustrative implementation, theaerial vehicle may be connected to a ground station via a tether. Whiletethered, the aerial vehicle can: (i) fly at a range of elevations andsubstantially along the path, and return to the ground, and (ii)transmit electrical energy to the ground station via the tether. (Insome implementations, the ground station may transmit electricity to theaerial vehicle for take-off and/or landing.)

In an AWT, an aerial vehicle may rest in and/or on a ground station (orperch) when the wind is not conducive to power generation. When the windis conducive to power generation, such as when a wind speed may be 3.5meters per second (m/s) at an altitude of 200 meters (m), the groundstation may deploy (or launch) the aerial vehicle. In addition, when theaerial vehicle is deployed and the wind is not conducive to powergeneration, the aerial vehicle may return to the ground station.

Moreover, in an AWT, an aerial vehicle may be configured for hoverflight and crosswind flight. Crosswind flight may be used to travel in amotion, such as a substantially circular motion, and thus may be theprimary technique that is used to generate electrical energy. Hoverflight in turn may be used by the aerial vehicle to prepare and positionitself for crosswind flight. In particular, the aerial vehicle couldascend to a location for crosswind flight based at least in part onhover flight. Further, the aerial vehicle could take-off and/or land viahover flight.

In hover flight, a span of a main wing of the aerial vehicle may beoriented substantially parallel to the ground, and one or morepropellers of the aerial vehicle may cause the aerial vehicle to hoverover the ground. In some implementations, the aerial vehicle mayvertically ascend or descend in hover flight. Moreover, in crosswindflight, the aerial vehicle may be oriented, such that the aerial vehiclemay be propelled by the wind substantially along a closed path, which asnoted above, may convert kinetic wind energy to electrical energy. Insome implementations, one or more rotors of the aerial vehicle maygenerate electrical energy by slowing down the incident wind.

During crosswind flight, the tether connecting the aerial vehicle to theground station may twist as the aerial vehicle orbits about an axisrelative to the ground station. In some implementations, the amount oftwist between the ground station end of the tether and the aerialvehicle end of the tether may vary based on a number of parametersduring crosswind flight. Twist in the tether may have beneficial ordetrimental effects on the system, depending on the system design andoperating parameters.

Embodiments described herein may allow for controlling the rotation andtwist of the tether for maximum benefit. In an illustrativeimplementation, a system may control the rotation, and amount of twist,of the tether when the tether is orbiting during crosswind flight of theaerial vehicle. In the case of a tether with electrical conductor(s), itmay be desirable to maintain the twist in the tether within a certainrange to reduce a strain of the conductors. Beneficially, such areduction of the strain may avoid breaking the conductors and/or mayimprove a fatigue life of the tether.

In some implementations, a system may include a tether, a tether gimbalassembly, a slip ring, a drive mechanism, and a control system. In anexample embodiment, the control system may be configured to operate thedrive mechanism to rotate the slip ring and the tether in order tocontrol the amount of twist in the tether. With this arrangement, theamount of twist in the tether during crosswind flight of the aerialvehicle may be actively controlled.

Moreover, in some implementations, a system may include a tether, atether gimbal assembly, a slip ring, and a resistive bearing system. Inan example embodiment, the resistive bearing system may be used topassively control tether twist during crosswind flight of the aerialvehicle. For example, the resistive bearing system may inhibit orprevent rotation of the slip ring and tether when the applied torquefrom a twisted tether is below a threshold level (which may be referredto as a slip limit). When the applied torque from a twisted tether isabove the slip limit, the bearing system may allow the slip ring andtether to rotate.

As another example, the resistive bearing system may be configured toallow the rotatable portion of the slip ring to rotate relative to thefixed portion and to provide a resistance (e.g., friction) to therotational torque of the tether so as to maintain the twist in thetether within a determined range of values.

Other embodiments may relate to methods for controlling rotation andtwist of a tether. For instance, some implementations may involvelaunching an aerial vehicle connected to a tether, transitioning theaerial vehicle to crosswind flight, and controlling, by a controlsystem, an amount of twist in the tether during crosswind flight.

II. ILLUSTRATIVE SYSTEMS

A. Airborne Wind Turbine (AWT)

FIG. 1 depicts an AWT 100, according to an example embodiment. Inparticular, the AWT 100 includes a ground station 110, a tether 120, andan aerial vehicle 130. As shown in FIG. 1, the tether 120 may beconnected to the aerial vehicle on a first end and may be connected tothe ground station 110 on a second end. In this example, the tether 120may be attached to the ground station 110 at one location on the groundstation 110, and attached to the aerial vehicle 130 at three locationson the aerial vehicle 130. However, in other examples, the tether 120may be attached at multiple locations to any part of the ground station110 and/or the aerial vehicle 130.

The ground station 110 may be used to hold and/or support the aerialvehicle 130 until it is in an operational mode. The ground station 110may also be configured to allow for the repositioning of the aerialvehicle 130 such that deploying of the device is possible. Further, theground station 110 may be further configured to receive the aerialvehicle 130 during a landing. The ground station 110 may be formed ofany material that can suitably keep the aerial vehicle 130 attachedand/or anchored to the ground while in hover flight, crosswind flight,and other flight modes, such as forward flight (which may be referred toas airplane-like flight). In some implementations, a ground station 110may be configured for use on land. However, a ground station 110 mayalso be implemented on a body of water, such as a lake, river, sea, orocean. For example, a ground station could include or be arranged on afloating off-shore platform or a boat, among other possibilities.Further, a ground station 110 may be configured to remain stationary orto move relative to the ground or the surface of a body of water.

In addition, the ground station 110 may include one or more components(not shown), such as a winch, that may vary a length of the tether 120.For example, when the aerial vehicle 130 is deployed, the one or morecomponents may be configured to pay out and/or reel out the tether 120.In some implementations, the one or more components may be configured topay out and/or reel out the tether 120 to a predetermined length. Asexamples, the predetermined length could be equal to or less than amaximum length of the tether 120. Further, when the aerial vehicle 130lands in the ground station 110, the one or more components may beconfigured to reel in the tether 120.

The tether 120 may transmit electrical energy generated by the aerialvehicle 130 to the ground station 110. In addition, the tether 120 maytransmit electricity to the aerial vehicle 130 in order to power theaerial vehicle 130 for takeoff, landing, hover flight, and/or forwardflight. The tether 120 may be constructed in any form and using anymaterial which may allow for the transmission, delivery, and/orharnessing of electrical energy generated by the aerial vehicle 130and/or transmission of electricity to the aerial vehicle 130. The tether120 may also be configured to withstand one or more forces of the aerialvehicle 130 when the aerial vehicle 130 is in an operational mode. Forexample, the tether 120 may include a core configured to withstand oneor more forces of the aerial vehicle 130 when the aerial vehicle 130 isin hover flight, forward flight, and/or crosswind flight. The core maybe constructed of any high strength fibers. In some examples, the tether120 may have a fixed length and/or a variable length. For instance, inat least one such example, the tether 120 may have a length of 140meters.

The aerial vehicle 130 may be configured to fly substantially along aclosed path 150 to generate electrical energy. The term “substantiallyalong,” as used in this disclosure, refers to exactly along and/or oneor more deviations from exactly along that do not significantly impactgeneration of electrical energy.

The aerial vehicle 130 may include or take the form of various types ofdevices, such as a kite, a helicopter, a wing and/or an airplane, amongother possibilities. The aerial vehicle 130 may be formed of solidstructures of metal, plastic and/or other polymers. The aerial vehicle130 may be formed of any material which allows for a highthrust-to-weight ratio and generation of electrical energy which may beused in utility applications. Additionally, the materials may be chosento allow for a lightning hardened, redundant and/or fault tolerantdesign which may be capable of handling large and/or sudden shifts inwind speed and wind direction.

The closed path 150 may be various different shapes in various differentembodiments. For example, the closed path 150 may be substantiallycircular. And in at least one such example, the closed path 150 may havea radius of up to 265 meters. The term “substantially circular,” as usedin this disclosure, refers to exactly circular and/or one or moredeviations from exactly circular that do not significantly impactgeneration of electrical energy as described herein. Other shapes forthe closed path 150 may be an oval, such as an ellipse, the shape of ajelly bean, the shape of the number of 8, etc.

The aerial vehicle 130 may be operated to travel along one or morerevolutions of the closed path 150. As shown in FIG. 1, the number ofrevolutions of the closed path 150 that the aerial vehicle 130 hastraveled along may be represented by N.

B. Illustrative Components of an AWT

FIG. 2 is a simplified block diagram illustrating components of the AWT200. The AWT 100 may take the form of or be similar in form to the AWT200. In particular, the AWT 200 includes a ground station 210, a tether220, and an aerial vehicle 230. The ground station 110 may take the formof or be similar in form to the ground station 210, the tether 120 maytake the form of or be similar in form to the tether 220, and the aerialvehicle 130 may take the form of or be similar in form to the aerialvehicle 230.

As shown in FIG. 2, the ground station 210 may include one or moreprocessors 212, data storage 214, and program instructions 216. Aprocessor 212 may be a general-purpose processor or a special purposeprocessor (e.g., digital signal processors, application specificintegrated circuits, etc.). The one or more processors 212 can beconfigured to execute computer-readable program instructions 216 thatare stored in a data storage 214 and are executable to provide at leastpart of the functionality described herein.

The data storage 214 may include or take the form of one or morecomputer-readable storage media that may be read or accessed by at leastone processor 212. The one or more computer-readable storage media caninclude volatile and/or non-volatile storage components, such asoptical, magnetic, organic or other memory or disc storage, which may beintegrated in whole or in part with at least one of the one or moreprocessors 212. In some embodiments, the data storage 214 may beimplemented using a single physical device (e.g., one optical, magnetic,organic or other memory or disc storage unit), while in otherembodiments, the data storage 214 can be implemented using two or morephysical devices.

As noted, the data storage 214 may include computer-readable programinstructions 216 and perhaps additional data, such as diagnostic data ofthe ground station 210. As such, the data storage 214 may includeprogram instructions to perform or facilitate some or all of thefunctionality described herein.

In a further respect, the ground station 210 may include a communicationsystem 218. The communication system 218 may include one or morewireless interfaces and/or one or more wireline interfaces, which allowthe ground station 210 to communicate via one or more networks. Suchwireless interfaces may provide for communication under one or morewireless communication protocols, such as Bluetooth, WiFi (e.g., an IEEE802.11 protocol), Long-Term Evolution (LTE), WiMAX (e.g., an IEEE 802.16standard), a radio-frequency ID (RFID) protocol, near-fieldcommunication (NFC), and/or other wireless communication protocols. Suchwireline interfaces may include an Ethernet interface, a UniversalSerial Bus (USB) interface, or similar interface to communicate via awire, a twisted pair of wires, a coaxial cable, an optical link, afiber-optic link, or other physical connection to a wireline network.The ground station 210 may communicate with the aerial vehicle 230,other ground stations, and/or other entities (e.g., a command center)via the communication system 218.

In an example embodiment, the ground station 210 may includecommunication systems 218 that allows for both short-range communicationand long-range communication. For example, the ground station 210 may beconfigured for short-range communications using Bluetooth and forlong-range communications under a CDMA protocol. In such an embodiment,the ground station 210 may be configured to function as a “hot spot”; orin other words, as a gateway or proxy between a remote support device(e.g., the tether 220, the aerial vehicle 230, and other groundstations) and one or more data networks, such as cellular network and/orthe Internet. Configured as such, the ground station 210 may facilitatedata communications that the remote support device would otherwise beunable to perform by itself.

For example, the ground station 210 may provide a WiFi connection to theremote device, and serve as a proxy or gateway to a cellular serviceprovider's data network, which the ground station 210 might connect tounder an LTE or a 3G protocol, for instance. The ground station 210could also serve as a proxy or gateway to other ground stations or acommand center, which the remote device might not be able to otherwiseaccess.

Moreover, as shown in FIG. 2, the tether 220 may include transmissioncomponents 222 and a communication link 224. The transmission components222 may be configured to transmit electrical energy from the aerialvehicle 230 to the ground station 210 and/or transmit electrical energyfrom the ground station 210 to the aerial vehicle 230. The transmissioncomponents 222 may take various different forms in various differentembodiments. For example, the transmission components 222 may includeone or more conductors that are configured to transmit electricity. Andin at least one such example, the one or more conductors may includealuminum and/or any other material which allows for the conduction ofelectric current. Moreover, in some implementations, the transmissioncomponents 222 may surround a core of the tether 220 (not shown).

The ground station 210 could communicate with the aerial vehicle 230 viathe communication link 224. The communication link 224 may bebidirectional and may include one or more wired and/or wirelessinterfaces. Also, there could be one or more routers, switches, and/orother devices or networks making up at least a part of the communicationlink 224.

Further, as shown in FIG. 2, the aerial vehicle 230 may include one ormore sensors 232, a power system 234, power generation/conversioncomponents 236, a communication system 238, one or more processors 242,data storage 244, and program instructions 246, and a control system248.

The sensors 232 could include various different sensors in variousdifferent embodiments. For example, the sensors 232 may include a globalpositioning system (GPS) receiver. The GPS receiver may be configured toprovide data that is typical of well-known GPS systems (which may bereferred to as a global navigation satellite system (GNNS)), such as theGPS coordinates of the aerial vehicle 230. Such GPS data may be utilizedby the AWT 200 to provide various functions described herein.

As another example, the sensors 232 may include one or more windsensors, such as one or more pitot tubes. The one or more wind sensorsmay be configured to detect apparent and/or relative wind. Such winddata may be utilized by the AWT 200 to provide various functionsdescribed herein.

Still as another example, the sensors 232 may include an inertialmeasurement unit (IMU). The IMU may include both an accelerometer and agyroscope, which may be used together to determine the orientation ofthe aerial vehicle 230. In particular, the accelerometer can measure theorientation of the aerial vehicle 230 with respect to earth, while thegyroscope measures the rate of rotation around an axis, such as acenterline of the aerial vehicle 230. IMUs are commercially available inlow-cost, low-power packages. For instance, the IMU may take the form ofor include a miniaturized MicroElectroMechanical System (MEMS) or aNanoElectroMechanical System (NEMS). Other types of IMUs may also beutilized. The IMU may include other sensors, in addition toaccelerometers and gyroscopes, which may help to better determineposition. Two examples of such sensors are magnetometers and pressuresensors. Other examples are also possible.

While an accelerometer and gyroscope may be effective at determining theorientation of the aerial vehicle 230, slight errors in measurement maycompound over time and result in a more significant error. However, anexample aerial vehicle 230 may be able to mitigate or reduce such errorsby using a magnetometer to measure direction. One example of amagnetometer is a low-power, digital 3-axis magnetometer, which may beused to realize an orientation independent electronic compass foraccurate heading information. However, other types of magnetometers maybe utilized as well.

The aerial vehicle 230 may also include a pressure sensor or barometer,which can be used to determine the altitude of the aerial vehicle 230.Alternatively, other sensors, such as sonic altimeters or radaraltimeters, can be used to provide an indication of altitude, which mayhelp to improve the accuracy of and/or prevent drift of the IMU. Inaddition, the aerial vehicle 230 may include one or more load cellsconfigured to detect forces distributed between a connection of thetether 220 to the aerial vehicle 230.

As noted, the aerial vehicle 230 may include the power system 234. Thepower system 234 could take various different forms in various differentembodiments. For example, the power system 234 may include one or morebatteries for providing power to the aerial vehicle 230. In someimplementations, the one or more batteries may be rechargeable and eachbattery may be recharged via a wired connection between the battery anda power supply and/or via a wireless charging system, such as aninductive charging system that applies an external time-varying magneticfield to an internal battery and/or charging system that uses energycollected from one or more solar panels.

As another example, the power system 234 may include one or more motorsor engines for providing power to the aerial vehicle 230. In someimplementations, the one or more motors or engines may be powered by afuel, such as a hydrocarbon-based fuel. And in such implementations, thefuel could be stored on the aerial vehicle 230 and delivered to the oneor more motors or engines via one or more fluid conduits, such aspiping. In some implementations, the power system 234 may be implementedin whole or in part on the ground station 210.

As noted, the aerial vehicle 230 may include the powergeneration/conversion components 236. The power generation/conversioncomponents 236 could take various different forms in various differentembodiments. For example, the power generation/conversion components 236may include one or more generators, such as high-speed, direct-drivegenerators. With this arrangement, the one or more generators may bedriven by one or more rotors. And in at least one such example, the oneor more generators may operate at full rated power wind speeds of 11.5meters per second at a capacity factor which may exceed 60 percent, andthe one or more generators may generate electrical power from 40kilowatts to 600 megawatts.

Moreover, as noted, the aerial vehicle 230 may include a communicationsystem 238. The communication system 238 may take the form of or besimilar in form to the communication system 218. The aerial vehicle 230may communicate with the ground station 210, other aerial vehicles,and/or other entities (e.g., a command center) via the communicationsystem 238.

In some implementations, the aerial vehicle 230 may be configured tofunction as a “hot spot”; or in other words, as a gateway or proxybetween a remote support device (e.g., the ground station 210, thetether 220, other aerial vehicles) and one or more data networks, suchas cellular network and/or the Internet. Configured as such, the aerialvehicle 230 may facilitate data communications that the remote supportdevice would otherwise be unable to perform by itself.

For example, the aerial vehicle 230 may provide a WiFi connection to theremote device, and serve as a proxy or gateway to a cellular serviceprovider's data network, which the aerial vehicle 230 might connect tounder an LTE or a 3G protocol, for instance. The aerial vehicle 230could also serve as a proxy or gateway to other aerial vehicles or acommand station, which the remote device might not be able to otherwiseaccess.

As noted, the aerial vehicle 230 may include the one or more processors242, the program instructions 246, and the data storage 244. The one ormore processors 242 can be configured to execute computer-readableprogram instructions 246 that are stored in the data storage 244 and areexecutable to provide at least part of the functionality describedherein. The one or more processors 242 may take the form of or besimilar in form to the one or more processors 212, the data storage 244may take the form of or be similar in form to the data storage 214, andthe program instructions 246 may take the form of or be similar in formto the program instructions 216.

Moreover, as noted, the aerial vehicle 230 may include the controlsystem 248. In some implementations, the control system 248 may beconfigured to perform one or more functions described herein. Thecontrol system 248 may be implemented with mechanical systems and/orwith hardware, firmware, and/or software. As one example, the controlsystem 248 may take the form of program instructions stored on anon-transitory computer readable medium and a processor that executesthe instructions. The control system 248 may be implemented in whole orin part on the aerial vehicle 230 and/or at least one entity remotelylocated from the aerial vehicle 230, such as the ground station 210.Generally, the manner in which the control system 248 is implemented mayvary, depending upon the particular application.

While the aerial vehicle 230 has been described above, it should beunderstood that the methods and systems described herein could involveany suitable aerial vehicle that is connected to a tether, such as thetether 220 and/or the tether 120.

C. Illustrative Aerial Vehicle

FIG. 3 depicts an aerial vehicle 330, according to an exampleembodiment. The aerial vehicle 130 and/or the aerial vehicle 230 maytake the form of or be similar in form to the aerial vehicle 330. Inparticular, the aerial vehicle 330 may include a main wing 331, pylons332 a, 332 b, rotors 334 a, 334 b, 334 c, 334 d, a tail boom 335, and atail wing assembly 336. Any of these components may be shaped in anyform which allows for the use of components of lift to resist gravityand/or move the aerial vehicle 330 forward.

The main wing 331 may provide a primary lift force for the aerialvehicle 330. The main wing 331 may be one or more rigid or flexibleairfoils, and may include various control surfaces, such as winglets,flaps (e.g., Fowler flaps, Hoerner flaps, split flaps, and the like),rudders, elevators, spoilers, dive brakes, etc. The control surfaces maybe used to stabilize the aerial vehicle 330 and/or reduce drag on theaerial vehicle 330 during hover flight, forward flight, and/or crosswindflight.

The main wing 331 and pylons 332 a, 332 b may be any suitable materialfor the aerial vehicle 330 to engage in hover flight, forward flight,and/or crosswind flight. For example, the main wing 331 and pylons 332a, 332 b may include carbon fiber and/or e-glass, and include internalsupporting spars or other structures. Moreover, the main wing 331 andpylons 332 a, 332 b may have a variety of dimensions. For example, themain wing 331 may have one or more dimensions that correspond with aconventional wind turbine blade. As another example, the main wing 331may have a span of 8 meters, an area of 4 meters squared, and an aspectratio of 15.

The pylons 332 a, 332 b may connect the rotors 334 a, 334 b, 334 c, and334 d to the main wing 331. In some examples, the pylons 332 a, 332 bmay take the form of, or be similar in form to, a lifting body airfoil(e.g., a wing). In some examples, a vertical spacing betweencorresponding rotors (e.g., rotor 334 a and rotor 334 b on pylon 332 a)may be 0.9 meters.

The rotors 334 a, 334 b, 334 c, and 334 d may be configured to drive oneor more generators for the purpose of generating electrical energy. Inthis example, the rotors 334 a, 334 b, 334 c, and 334 d may each includeone or more blades, such as three blades or four blades. The rotorblades may rotate via interactions with the wind and be used to drivethe one or more generators. In addition, the rotors 334 a, 334 b, 334 c,and 334 d may also be configured to provide thrust to the aerial vehicle330 during flight. With this arrangement, the rotors 334 a, 334 b, 334c, and 334 d may function as one or more propulsion units, such as apropeller. Although the rotors 334 a, 334 b, 334 c, and 334 d aredepicted as four rotors in this example, in other examples the aerialvehicle 330 may include any number of rotors, such as less than fourrotors or more than four rotors.

A tail boom 335 may connect the main wing 331 to the tail wing assembly336, which may include a tail wing 336 a and a vertical stabilizer 336b. The tail boom 335 may have a variety of dimensions. For example, thetail boom 335 may have a length of 2 meters. Moreover, in someimplementations, the tail boom 335 could take the form of a body and/orfuselage of the aerial vehicle 330. In such implementations, the tailboom 335 may carry a payload.

The tail wing 336 a and/or the vertical stabilizer 336 b may be used tostabilize the aerial vehicle 330 and/or reduce drag on the aerialvehicle 330 during hover flight, forward flight, and/or crosswindflight. For example, the tail wing 336 a and/or the vertical stabilizer336 b may be used to maintain a pitch of the aerial vehicle 330 duringhover flight, forward flight, and/or crosswind flight. The tail wing 336a and the vertical stabilizer 336 b may have a variety of dimensions.For example, the tail wing 336 a may have a length of 2 meters.Moreover, in some examples, the tail wing 336 a may have a surface areaof 0.45 meters squared. Further, in some examples, the tail wing 336 amay be located 1 meter above a center of mass of the aerial vehicle 330.

While the aerial vehicle 330 has been described above, it should beunderstood that the systems and methods described herein could involveany suitable aerial vehicle that is connected to an airborne windturbine tether, such as the tether 120 and/or the tether 220.

D. Illustrative Tether Twist

FIGS. 4a-c depict twist in a tether 420, according to an exampleembodiment. The tether 120 and/or the tether 220 may take the form of orbe similar in form to the tether 420. Referring to FIG. 4a , the tether420 includes a bridal portion 421, a proximate tether end 422, a distaltether end 424, and a long axis 426 that extends between the proximatetether end 422 and the distal tether end 424. In the illustratedexample, the distal tether end 424 is coupled to the aerial vehicle 330.The proximate tether end 422 may be coupled to a ground station (notshown), such as the ground station 110 and/or the ground station 210. Inaddition, the tether 420 may include at least one insulated electricalconductor (not shown) coupled to the aerial vehicle 330. FIGS. 4a-c ,and remaining Figures depicting tethers, are for illustrative purposesonly and may not reflect all components or connections. Further, asillustrations the Figures may not reflect actual operating conditions,but are merely to illustrate embodiments described. For example, while astraight cylinder may be used to illustrate the described tetherembodiments, during orbiting crosswind flight the tether may in practiceexhibit some level of droop between the ground station and the aerialvehicle. Further still, the relative dimensions in the Figures may notbe to scale, but are merely to illustrate the embodiments described.

FIGS. 4a-c illustrate twist in the tether 420 between the proximatetether end 422 and the distal tether end 424 as the aerial vehicle 330travels along a closed path, such as the closed path 150. In someembodiments, an amount of twist in the tether 420 may be measured as anangular distance between a point α on the tether 420 at the distaltether end 424 and a point α′ on the tether 420 at the proximate tetherend 422. Other measurement points are also possible. For example, anamount of twist may be at two or more points located between the distaltether end 424 and the proximate tether end 422. As shown in FIGS. 4a-c, an amount of twist in the tether 420 may increase as the number ofrevolutions of the closed path, N, that the aerial vehicle 330 hastraveled along increases.

For example, as shown in FIG. 4a , when N=0, an illustrative referenceline 428 on the tether 420 may extend between the point α and the pointα′ that is substantially parallel to the long axis 426. With thisarrangement, the angular distance between the point α and the point α′may be substantially zero. Accordingly, the amount of twist in thetether 420 may be substantially zero.

The term “substantially parallel,” as used in the disclosure, refers toexactly parallel or one or more deviations from exactly parallel that donot significantly impact controlling rotation and twist of a tether asdescribed herein. In addition, the term “substantially zero,” as used inthis disclosure, refers to exactly zero or one or more deviations fromzero that do not significantly impact controlling rotation and twist ofa tether as described herein.

As shown in FIG. 4b , after the aerial vehicle 330 completes one orbit,and thus N=1, the tether may twist about the long axis 426. Thusreference line 428 may form a helix around the long axis 426. With thisarrangement, when N=1, the angular distance between the point α and thepoint α′ may be greater than the angular distance between the point αand the point α′ when N=0. Accordingly, when N=1, an amount of twist inthe tether 420 may be greater than an amount of twist in the tether 420when N=0.

Further, as shown in FIG. 4c , after the aerial vehicle 330 completestwo orbits, and thus N=2, the tether may further twist about the longaxis 426. In the illustrated example, the helical pitch of referenceline 428 may be greater than the helical pitch of the reference line 428in FIG. 4b . With this arrangement, when N=2, the angular distancebetween the point α and the point α′ may be greater than the angulardistance between the point α and the point α′ when N=1. Accordingly,when N=2, an amount of twist in the tether 420 may be greater than anamount of twist in the tether 420 when N=1.

E. Aerial Vehicle Coupled to a Ground Station Via a Tether

FIG. 5a depicts the aerial vehicle 330 coupled to a ground station 510via the tether 420, according to an example embodiment. Referring toFIG. 5a , the ground station 510 may include a winch drum 512 and aplatform 514. The ground station 110 and/or the ground station 210 maytake the form of or be similar in form to the ground station 510. FIG.5a is for illustrative purposes only and may not reflect all componentsor connections.

As shown in FIG. 5a , the tether 420 may be coupled to a tether gimbalassembly 542 at the proximate tether end 422 and to the aerial vehicle330 at the distal tether end 424. Moreover, as shown in FIG. 5a , thetether gimbal assembly 542 may also be coupled to the winch drum 512which in turn may be coupled to the platform 514. A slip ring 544located between the tether 420 and the tether gimbal assembly 542 mayallow the tether 420 to rotate about the long axis 426 of the tether 420(as shown in, and described with respect to, FIGS. 4a-c ) relative tothe ground station 510.

In some embodiments, the tether gimbal assembly 542 may be configured torotate about one or more axes, such as a horizontal axis 552 and anazimuth axis 554, in order to allow the proximate tether end 422 to movein those axes in response to movement of the aerial vehicle 330.Moreover, in some embodiments, the slip ring 544 may include a fixedportion 544 a, a rotatable portion 544 b, and one or more insulatedelectrically conductive pathways (not shown). The rotatable portion 544b may be coupled to the tether 420. The fixed portion 544 a may becoupled to the tether gimbal assembly 542. The one or more insulatedelectrically conductive pathways may provide an electrical connectionbetween one or more electrical conductors in the tether, and one or moreground-side electrical connections (not shown).

The use of the word fixed in the fixed portion 544 a of the slip ring544 is not intended to limit fixed portion 544 a to a stationaryconfiguration. In this example, the fixed portion 544 a may move in axesdescribed by the tether gimbal assembly 542 (e.g., the horizontal axis552 and azimuth 554), and may rotate about the ground station 510 as thewinch drum 512 rotates, but the fixed portion 544 a will not rotateabout the tether 420, i.e., with respect to the long axis 426 of thetether. Moreover, in this example, the rotatable portion 544 b of theslip ring 544 may be coupled to the tether gimbal assembly 542 andconfigured to substantially rotate with the rotation of tether 420.

As shown in FIG. 5a , a drive mechanism 546 may be coupled to therotatable portion 544 b and configured to rotate the rotatable portion544 b (and consequently the proximate tether end 422) relative to thestationary portion 544 a. As an example, the drive mechanism 546 mayinclude a servo motor.

Via the slip ring 544, the tether 420 may rotate about its centerlinealong the long axis 426 as the aerial vehicle 330 orbits. The distaltether end 424 may rotate a different amount than the proximate tetherend 422, resulting in an amount of twist along the length of the tether420. With this arrangement, the amount of twist in the tether 420 mayvary based on a number of parameters during crosswind flight of theaerial vehicle 330.

In a further aspect, the slip ring 544 may not be coupled to the tethergimbal assembly 542. For example, as shown in FIG. 5b , the slip ring544 may be near the platform 514. As such, the tether 420 may passthrough the tether gimbal assembly 542. In the illustrated example, thefixed portion 544 a of the slip ring 544 may be coupled to platform 514,the winch drum 512, or another component of the ground station 510 andthe tether 420 may be coupled to the rotatable portion 544 b of the slipring 544 at the proximate tether end 422. The other connections of theaerial vehicle 330, the winch drum 512, the platform 514, the tethergimbal assembly 542, and the drive mechanism 546, as well as otherconnections, may be described with respect to FIG. 5 a.

In some embodiments, a flexible coupling 548 may be used to route thetether 420 from the tether gimbal assembly 542 to the slip ring 544. Asshown in FIG. 5b , the flexible coupling 548 includes a first end 548 aand a second end 548 b. The first end 548 a of the flexible coupling 548may be coupled to the tether gimbal assembly 542 and the second end 548b of the flexible coupling 548 may be coupled to the rotatable portion544 b of the slip ring 544.

Moreover, in some embodiments, the tether 420 may be coupled to thetether gimbal assembly 542 at the proximate tether end 422, and one ormore cables (or wires) may be connected to the proximate tether end 422.The one or more cables may connect the tether 420 to the slip ring 544.

F. Systems for Controlling Rotation and Twist of a Tether

FIG. 6a depicts a system 600 for controlling rotation and twist in thetether 420, according to an example embodiment. In particular, thesystem 600 includes a control system 650. Referring to FIG. 6a , thetether 420 may be coupled to a tether gimbal assembly 542 at theproximate tether end 422 and to the aerial vehicle 330 at the distaltether end 424. Additionally or alternatively, the tether 420 may passthrough the tether gimbal assembly 542. Moreover, as shown in FIG. 6a ,the tether gimbal assembly 542 may be coupled to the winch drum 512which in turn may be coupled to the platform 514, the rotatable portion544 b of the slip ring 544 may be coupled to the tether 420, the fixedportion 544 a of the slip ring 544 may be coupled to the tether gimbalassembly 542, and drive mechanism 546 may be coupled to the rotatableportion 544 b. For example, the tether 420, the slip ring 544, thetether gimbal assembly 542 connections, as well as other connections,may be as described with respect to FIG. 5 a.

Alternatively, the fixed portion 544 a of the slip ring 544 may becoupled to the platform 514, the winch drum 512, or another component ofthe ground station 510 as described with reference to FIG. 5b . Forexample, the tether 420, the slip ring 544, the tether gimbal assembly542 connections, as well as other connections, may be described withrespect to FIG. 5 b.

The control system 650 is configured to control operation(s) of thesystem 600 and its components. In some embodiments, the control system650 may be configured to perform one or more functions described herein.For example, in some embodiments, the control system 650 may beconfigured to operate the drive mechanism 546 to control twist in thetether 420. In the illustrated embodiment, the control system 650 isconnected to at least the drive mechanism 546, though other alternativeor additional connections are possible, including but not limited to thetether 420, the slip ring 544, and the aerial vehicle 330. With thisarrangement, an amount of twist in the tether 420 during crosswindflight of the aerial vehicle 330 may be actively controlled. In someexamples, the control system 650 may be connected to at least onecomponent by a wired connection or a wireless connection.

The control system 650 may be similar in form to the control system 248.For instance, the control system 650 may be implemented with mechanicalsystems and/or with hardware, firmware, and/or software. As one example,the control system 650 may take the form of program instructions storedon a non-transitory computer readable medium and a processor thatexecutes the instructions. The control system 650 may be implemented inwhole or in part on the ground station 510 and/or at least one entityremotely located from the ground station, such as the aerial vehicle330. Generally, the manner in which the control system 650 isimplemented may vary, depending upon the particular application.

FIG. 6b depicts a foreshortened view of the tether 420, according to anexample embodiment. As noted, in some embodiments, an amount of twist Tin the tether 420 may be measured as an angular distance between a pointα on the tether 420 at the distal tether end 424 and a point α′ on thetether 420 at the proximate tether end 422. Alternatively oradditionally, the amount of twist in the tether may be measured betweenpoints along the tether other than a and α′. For example, the amount oftwist may be measured along a portion of the tether 420 near theproximate end 422 or the distal end 424, or over multiple portions ofthe tether 420. In any case, the control system 650 may be configured tooperate the drive mechanism 546 to control the amount of twist.

Further, in some embodiments, it may be desirable for the twist in thetether 420 to be positive. This may be accomplished by maintaining arate of rotation in the proximate tether end 422 via the drive mechanism546 such that the proximate tether end 422 is twisted a fixed orvariable amount towards the direction of aerial vehicle 330 orbit beyonda natural state of the tether 420 (for example, when no torque ortension is applied via a drive mechanism and the proximate end 422 isallowed to rotate freely via a free-running slip ring). This may bereferred to as a lead mode. In such embodiments, the control system 650may be configured to operate the drive mechanism 546 in the lead mode.

Further still, in some embodiments, it may be desirable for the twist inthe tether 420 to be negative. This may be accomplished by maintaining arate of rotation in the proximate tether end 422 via the drive mechanism546 such that the proximate tether end 422 is twisted a fixed orvariable amount away from the direction of rotation, although theproximate tether end 422 may still be rotating in the direction of theaerial vehicle 330 orbit. This may referred to as a lag mode. In suchembodiments, the control system 650 may be configured to operate thedrive mechanism 546 in the lag mode.

In addition, in some embodiments, the control system 650 may beconfigured to operate the drive mechanism at variable speeds, fixedspeeds, or in an on/off fashion in order to maintain the desired twistwithin a certain operating range. For example, the control system 650may be configured to maintain the tether 420 twist within a range ofvalues by activating and deactivating the drive mechanism 546 (e.g.,pulsing a drive motor attached to the slip ring). As another example,the control system 650 may be configured to maintain the tether 420twist within a range of values by causing the drive mechanism 546 torotate the rotatable portion 544 b at a constant rate. As yet anotherexample, the control system 650 may be configured to maintain the tether420 twist within a range of values by causing the drive mechanism 546 torotate the rotatable portion 544 b at a variable rate. In such examples,the variable rate may be determined in reference to at least therotational rate of the tether 420. For instance, in at least one suchexample, the variable rate may be determined in reference to at leastthe rotational rate of the distal tether end 424 or a rotational speedof the aerial vehicle 330. Further, in at least one such example, thevariable rate may be determined in reference to at least the rotationalrate of the proximate tether end 422.

Moreover, in some embodiments, the control system 650 may be configuredto determine one or more operational or environmental parameters thataffect an AWT, such as AWT 100 and/or AWT 200, and then control theamount of twist in the tether 420 based at least in part on thedetermined parameter. As examples, the parameters may include tether 420tension, position of the aerial vehicle 330, load(s) on the aerialvehicle 330, velocities of the aerial vehicle 330, wind speed(s),temperature of a tether 420 conductor, environmental temperature,conductor resistance, and/or current flowing in a conductor. Forexample, by increasing or decreasing the twist in the tether 420,tension in the tether 420 can be increased or decreased. And in at leastone such embodiment, when the tether 420 includes two or more layers, itmay desirable to maintain a relative tension between the layers of thetether 420. The control system 650 may determine the parameters at leastin part by information provided by any of the sensors 232 of the aerialvehicle 230.

Although system 600 has been described above, other example systems arepossible as well. For example, although the drive mechanism 546 iscoupled to the rotatable portion 544 b of the slip ring 544 in thesystem 600, in other example systems the drive mechanism 546 may not becoupled to the rotatable portion 544 b. Instead, in some embodiments,the drive mechanism 546 may be coupled to a portion of the tether 420.In such embodiments, the drive mechanism 546 may be configured to rotatethe coupled portion of the tether 420.

Although in system 600 the slip ring 544 is coupled to the tether gimbalassembly 542 or the ground station 510, in other example systems theslip ring 544 may be coupled instead to the aerial vehicle 330. Forinstance, in some embodiments, the fixed portion 544 a of the slip ring544 may be coupled to the aerial vehicle 330, the rotatable portion 544b of the slip ring 544 may be coupled to the distal tether end 424, andthe drive mechanism 546 may be coupled to the rotatable portion 544 b.Moreover, in an example system where the slip ring 544 is coupled to theaerial vehicle 330, the system may not include the tether gimbalassembly 542. Instead, in some embodiments, the proximate tether end 422may be coupled to the winch drum 512.

As yet another example, although system 600 includes the drive mechanism546, other example systems may include two or more drive mechanismscoupled to the slip ring 544. Beneficially, such redundancy may improvethe reliability of the system. In some embodiments, each drive mechanismof the two or more drive mechanisms may take the form of or be similarin form to the drive mechanism 546.

FIG. 7 depicts another system 700 for controlling rotation and twist ofthe tether 420, according to an example embodiment. In particular, thesystem 700 includes a resistive bearing system 760. The resistivebearing system 760 may passively control an amount of twist in thetether 420 during crosswind flight. Referring to FIG. 7, the tether 420may be coupled to the tether gimbal assembly 542 at the proximate tetherend 422 and to the aerial vehicle 330 at the distal tether end 424.Additionally or alternatively, the tether 420 may pass through thetether gimbal assembly 542. Moreover, as shown in FIG. 7, the tethergimbal assembly 542 may be coupled to the winch drum 512 which in turnis coupled to the platform 514, the rotatable portion 544 b of the slipring 544 may be coupled to the tether 420, the fixed portion 544 a ofthe slip ring 544 may be coupled to the tether gimbal assembly 542, andthe resistive bearing system 760 may be coupled to the slip ring 544and/or the tether gimbal assembly 542.

Alternatively, the fixed portion 544 a of the slip ring 544 may becoupled to the platform 514, the winch drum 512, or another component ofthe ground station 510. For example, the tether 420, the slip ring 544,the tether gimbal assembly 542 connections, as well as otherconnections, may be described with respect to FIG. 5 b.

In some embodiments, the resistive bearing system 760 may be configuredto allow the rotatable portion 544 b to rotate relative to the fixedportion 544 a when a torque provided by the tether 420 exceeds a sliplimit, and may be further configured to inhibit the rotation of therotatable portion 544 b relative to the fixed portion 544 a when thetorque provided by the tether 420 does not exceed the slip limit. Inother embodiments, the resistive bearing system 760 may be configured toallow the proximate tether end 422 to rotate when a torque at theproximate tether end 422 exceeds a slip limit, and may be furtherconfigured to inhibit the rotation of the proximate tether end 422 whenthe torque does not exceed the slip limit.

Moreover, in some embodiments, the slip limit may be based on any of theparameters of the tether 420, the aerial vehicle 330, and/or theenvironment as described herein. Further, in some embodiments, thetether 420 may include fibers (not shown) at a lay angle that is lessthan any helical lay angle of conductor(s) of the tether 420. As such,the fibers may provide torque to drive or assist driving the resistivebearing system 760. Further still, in some embodiments, the tether 420may include fibers at a lay angle that is equal to or greater than anyhelical lay angle of conductor(s) of the tether 420. As such, the fibersmay provide torque to drive or assist driving the resistive bearingsystem 760.

Further, in some embodiments, the resistive bearing system 760 mayinclude a brake (not shown) and the brake may be configured to inhibitthe rotation of the rotatable portion 544 b relative to the fixedportion 544 a, for example, when the torque provided by the tether 420does not exceed the slip limit.

Further still, in some embodiments, the resistive bearing system 760 maybe configured to allow the rotatable portion 544 b of the slip ring 544to rotate relative to the fixed portion 544 a and to provide aresistance to the rotational torque of the tether 420 so as to maintainthe twist in the tether 420 within a determined range of values.Moreover, in some embodiments, the resistance to the rotational torqueof the tether 420 provided by the resistive bearing system 760 may bebased on any of the parameters of the tether 420, the aerial vehicle330, and/or the environment as described herein. In other embodiments,the resistive bearing system 760 may be configured to allow theproximate tether end 422 to rotate and to provide a resistance to therotational torque of the tether 420 so as to maintain the twist in thetether 420 within a determined range of values.

In addition, in some embodiments, a resistance of the resistive bearingsystem 760 may vary based on any parameters of the tether 420, theaerial vehicle 330, and/or the environment as described herein. Forexample, a friction brake may be used to vary the resistance of theresistance bearing system 760.

Although system 700 has been described above, other example systems arepossible as well. For example, although the resistive bearing system 760is coupled to the slip ring 544 and/or the tether gimbal assembly 542 inthe system 700, in other example systems the resistive bearing system760 may not be coupled to the slip ring 544 and/or the tether gimbalassembly 542. Instead, in some embodiments, the resistive bearing system760 may be coupled to a portion of the tether 420. In such embodiments,the resistive bearing system 760 may be configured to allow the coupledportion of the tether 420 to rotate when a torque at the coupled portionof the tether 420 exceeds a slip limit, and may be further configured toinhibit the rotation of the coupled portion of the tether 420 when thetorque does not exceed the slip limit. Alternatively, in suchembodiments, the resistive bearing system 760 may be configured to allowthe coupled portion of the tether 420 to rotate and to provide aresistance to the rotational torque of the tether 420 so as to maintainthe twist in the tether 420 within a determined range of values.

G. Illustrative Tethers

FIG. 8 depicts a cross-section of a tether 820, according to an exampleembodiment. The tether 120, the tether 220, and/or the tether 420 maytake the form of or be similar in form to the tether 820. The tether 820includes a core 872, at least one compliant layer 874, and at least oneconductor 876. As shown in FIG. 8, the compliant layer 874 is locatedbetween the core 872 and the conductor 876. The core 872 may beconfigured to withstand a strain load, for example, of between 0.8% and1.0%. In some embodiments, the conductor 876 may be configured towithstand less strain than the core 872.

The conductor 876 may be helically wound around a length of the core872. With this arrangement, strain on the conductor 876 may be reducedduring normal operation. In addition, when the tether 820 is twisted,the conductor 876 may compress into the compliant layer 874. With thisarrangement, strain on the conductor 876 may be further reduced.

When the tether 820 is twisted, a helically wound conductor 876 may bein tension or compression. For example, when the direction of twist ofthe tether 820 corresponds to the conductor's 876 helical winding, theconductor 876 may be in tension. And when the direction of the twist isin opposition to the conductor's 876 helical winding, the conductor 876may be in compression.

Although the tether 820 has been described above as including theconductor 876, other example tethers may include a conductor layerhaving two or more conductors. In some embodiments, each conductor ofthe two or more conductors may take the form of or be similar in form tothe conductor 876. Moreover, in some embodiments, each conductor of thetwo or more conductors may be helically wound around a length of thecore 872.

Further, although the tether 820 has been described above as includingthe compliant layer 874, other example tethers may not include acomplaint layer.

FIG. 9 depicts a cross-section of another tether 920, according to anexample embodiment. The tether 120, the tether 220, and/or the tether420 may take the form of or be similar in form to the tether 920. Inparticular, the tether 920 includes a core layer 972 having two or morecore elements 973. The tether 920 may include the core layer 972, acomplaint layer 974, and at least one conductor 976. The compliant layer974 may take the form of or be similar in form to the compliant layer874, and the conductor 976 may take the form of or be similar in form tothe conductor 876.

As shown in FIG. 9, the complaint layer 974 is located between the corelayer 972 and the conductor 976. The conductor 976 may be helicallywound around a length of the core layer 972 in the same or similar wayas the conductor 876 may be helically wound around a length of the core872 in the tether 820. In addition, the tether 920 may include two ormore conductors in the same or similar way as the tether 820 may includetwo or more conductors.

As noted, the core layer 972 includes two or more core elements 973. Inthe illustrated example, the two or more core elements 973 may includeseven core elements: a first core element 973 a, a second core element973 b, a third core element 973 c, a fourth core element 973 d, a fifthcore element 973 e, a six core element 973 f, and a seventh core element973 g. However, in other examples, the two or more core elements 973 mayinclude more than seven core elements or less than seven core elements.Moreover, in some embodiments, each core element may be the same orsimilar. However, in some embodiments, at least one core element mayhave a different material, thickness, length, lay angle, etc.

Further, in some embodiments, at least one core element may be helicallywound around a length of the tether 920. With this configuration, thecore layer 972 may have a lower polar moment of inertia than a polarmoment of inertia of the core 872. Beneficially, the core layer 972 mayallow for a greater amount of twist in the tether 920 than allowed bycore 872 in tether 820. Further still, in some embodiments, at least onecore element may include a carbon rod and the core layer 972 may beconfigured to provide torque to drive a resistive bearing system, suchas the resistive bearing system 760.

FIG. 10 depicts a cross-section of yet another tether 1020, according toan example embodiment. In particular, the tether 1020 includes a torquelayer 1078 having at least one fiber 1079. The tether 120, the tether220, and/or the tether 420 may take the form of or be similar in form tothe tether 1020. The tether 1020 includes a core 1072, a complaint layer1074, a conductor layer 1076, and the torque layer 1078. The conductorlayer 1076 may include at least one conductor 1077. The core 1072 maytake the form of or be similar in form to the core 872 or the core layer972, the complaint layer 1074 may take the form of or be similar in formto the complaint layer 874 and/or the complaint layer 974, and theconductor 1077 may take the form of or be similar in form to theconductor 876 and/or the conductor 976.

As shown in FIG. 10, the complaint layer 1074 is located between thecore 1072 and the conductor layer 1076. Moreover, as shown in FIG. 10,the core 1072, the complaint layer 1074, and the conductor layer 1076may be located inside of the torque layer 1078. The conductor 1077 maybe helically wound around a length of the core 1072 in the same orsimilar way as the conductor 876 may be helically wound around a lengthof the core 872 in the tether 820 and the conductor 976 may be helicallywound around a length of the core layer 972 of the tether 920. Inaddition, the tether 1020 may include two or more conductors in the sameor similar way as the tether 820 may include two or more conductors andthe tether 920 may include two or more conductors.

As noted, the tether 1020 includes the torque layer 1078 having thefiber 1079. In some embodiments, the fiber 1079 may be helically woundaround a length of the tether 1020 over the conductor layer 1076. Withthis configuration, the fiber 1079 may be configured to provide torqueto drive a resistive bearing system, such as the resistive bearingsystem 760. As an example, the fiber 1079 may include carbon or anysuitable material configured to drive the resistive bearing system.

In such embodiments, the fiber 1079 may be helically wound in thedirection that an aerial vehicle, such as the aerial vehicle 130, theaerial vehicle 230, and/or the aerial vehicle 330, rotates duringcrosswind flight (e.g., right-handed direction). As the tether twists,the helically wound fiber 1079 will create torque by virtue of awinding/unwinding force. Further, in such embodiments, a lay angle ofthe fiber 1079 may be based at least in part on one or more parameters,including friction in the resistive bearing system, stiffness of thefiber 1079, the compressibility (e.g., bulk modulus) of the tether 1020,allowable strain in the conductor 1077, and alternating tension of thetether 1020. Further still, in such embodiments, a lay angle of thefiber 1079 may be less than a lay angle of the conductor 1077.

Moreover, in some embodiments, at least one parameter of the torquelayer 1078 may be selected so as to increase or decrease a tensilestrength of the tether 1020. In such embodiments, at least one parameterof the fiber 1079 may be selected so as to increase or decrease atensile strength of the tether 1020. Further, in some embodiments, atleast one parameter of the torque layer 1078 may be selected so as toincrease or decrease a stiffness of the tether 1020. In suchembodiments, at least one parameter of the fiber 1079 may be selected soas to increase or decrease a stiffness of the tether 1020.

Although the torque layer 1078 has been described above as having thefiber 1079, other example tethers may have two or more fibers. In someembodiments, each fiber of the two or more fibers may take the form ofor be similar in form to the fiber 1079. Moreover, in some embodiments,each fiber of the two or more fibers may be helically wound around alength of the tether 1020. Further, in such embodiments, a correspondinglay angle of each fiber of the two or more fibers may be less than a layangle of the conductor 1020.

Moreover, although the tether 1020 has been described above with thecore 1072, the compliant layer 1074, and the conductor layer 1076 beinglocated inside of the torque layer 1078, in other example tethers, thetorque layer 1078 may be located between the core layer 1072 and theconductor layer 1076 (e.g., between the compliant layer 1074 and theconductor 1077). In some embodiments, the fiber 1079 may be helicallywound around a length of the tether 1020 over the core 1072.

III. ILLUSTRATIVE METHODS

FIG. 11 is a flowchart illustrating a method 1100, according to anexample embodiment. Illustrative methods, such as method 1100, may becarried out in whole or in part by a component or components of an AWT,such as by the one or more components of the AWT 100 shown in FIG. 1,and the AWT 200 shown in FIG. 2.

As shown by block 1102, the method 1100 may involve launching an aerialvehicle connected to a tether. The aerial vehicle may take form of or besimilar in form to the aerial vehicle 130, the aerial vehicle 230,and/or the aerial vehicle 330. The tether may take the form of or besimilar in form to the tether 120, the tether 220, the tether 420, thetether 820, the tether 920, and the tether 1020.

As shown by block 1104, the method 1100 may involve transitioning theaerial vehicle to crosswind flight. In some embodiments, the aerialvehicle may transition to crosswind flight via hover flight and/orforward flight.

As shown by block 1106, the method 1100 may involve controlling, by acontrol system, an amount of twist in the tether during crosswindflight. The control system may take the form of or be similar in form tothe control system 248 and/or the control system 650.

In some embodiments, a drive mechanism is coupled to the tether, andcontrolling, by the control system, the amount of twist in the tetherduring crosswind flight may involve operating a drive mechanism in a lagmode. Moreover, in some embodiments, a drive mechanism is coupled to thetether, and controlling, by the control system, the amount of twist inthe tether during crosswind flight may involve operating a drivemechanism in a lead mode. Further, in some embodiments, a drivemechanism is coupled to the tether, and controlling, by the controlsystem, the amount of twist in the tether during crosswind flight mayinvolve activating and deactivating the drive mechanism.

Further still, in some embodiments, a rotatable portion of a slip ringis coupled to the tether, a drive mechanism is coupled to the rotatableportion, and controlling, by the control system, the amount of twist inthe tether during crosswind flight may involve causing the drivemechanism to rotate a rotatable portion of the slip ring coupled to thetether at a constant rate.

Moreover, in some embodiments, a rotatable portion of a slip ring iscoupled to the tether, a drive mechanism is coupled to the rotatableportion, and controlling, by the control system, the amount of twist inthe tether during crosswind flight may involve causing the drivemechanism to rotate the rotatable portion of the slip ring at a variablerate. And in at least one such embodiment, the variable rate may bedetermined in reference to at least the rotational rate of the tether.

Further, in some embodiments, a drive mechanism is coupled to a tether,and the method 1100 may further involve determining the value of anoperational or environmental parameter and operating the drive mechanismto control tether twist based at least in part on the determinedoperational or environmental parameter. And in at least one suchembodiment, the operational or environmental parameter comprises atension on the tether, a load on the aerial vehicle, a position of theaerial vehicle, a velocity of the aerial vehicle, a wind speed, atemperature of the at least one conductor, an environmental temperature,a resistance of the at least one conductor, or the amount of electricalcurrent carried by the at least one conductor.

IV. CONCLUSION

The particular arrangements shown in the Figures should not be viewed aslimiting. It should be understood that other embodiments may includemore or less of each element shown in a given Figure. Further, some ofthe illustrated elements may be combined or omitted. Yet further, anexemplary embodiment may include elements that are not illustrated inthe Figures.

Additionally, while various aspects and embodiments have been disclosedherein, other aspects and embodiments will be apparent to those skilledin the art. The various aspects and embodiments disclosed herein are forpurposes of illustration and are not intended to be limiting, with thetrue scope and spirit being indicated by the following claims. Otherembodiments may be utilized, and other changes may be made, withoutdeparting from the spirit or scope of the subject matter presentedherein. It will be readily understood that the aspects of the presentdisclosure, as generally described herein, and illustrated in theFigures, can be arranged, substituted, combined, separated, and designedin a wide variety of different configurations, all of which arecontemplated herein.

What is claimed is:
 1. A system comprising: a tether comprising: adistal tether end coupled to an aerial vehicle; a proximate tether end;and at least one insulated electrical conductor coupled to the aerialvehicle, wherein the tether has an amount of twist between the distaltether end and the proximate tether end; a tether gimbal assembly,wherein the tether gimbal assembly is coupled to the tether and isrotatable about at least a horizontal axis or an azimuth axis; a drivemechanism coupled to the tether at the proximate tether end, wherein thedrive mechanism rotates the tether about a long axis of the tether thatextends between the distal tether end and the proximate tether end; anda control system, wherein the control system operates the drivemechanism to change the amount of twist in the tether.
 2. The system ofclaim 1, wherein the control system operates the drive mechanism in alag mode.
 3. The system of claim 1, wherein the control system operatesthe drive mechanism in a lead mode.
 4. The system of claim 1, whereinthe control system operates the drive mechanism by activating anddeactivating the drive mechanism.
 5. The system of claim 1, wherein thecontrol system operates the drive mechanism by causing the drivemechanism to rotate the tether at a constant rate.
 6. The system ofclaim 1, wherein the control system operates the drive mechanism bycausing the drive mechanism to rotate the tether at a variable rate. 7.A system comprising: a tether comprising: a distal tether end coupled toan aerial vehicle; a proximate tether end; and at least one insulatedelectrical conductor coupled to the aerial vehicle; a tether gimbalassembly, wherein the tether gimbal assembly is coupled to the tetherand is rotatable about at least a horizontal axis or an azimuth axis;and a resistive bearing system coupled to the tether gimbal assembly,wherein the resistive bearing system is configured to allow theproximate tether end to rotate when a torque at the proximate tether endexceeds a slip limit, and further configured to inhibit the rotation ofthe proximate tether end when the torque does not exceed the slip limit.8. The system of claim 7, wherein the resistive bearing system comprisesa brake.
 9. The system of claim 7, wherein the resistive bearing systemcomprises a friction brake.
 10. The system of claim 7, wherein the sliplimit is based at least in part on a tension of the tether.
 11. Thesystem of claim 7, wherein the tether further comprises a torque layerhaving at least one fiber, wherein the at least one fiber is helicallywound around a length of the tether over the at least one insulatedconductor, and wherein the at least one fiber is configured to provide atorque to drive the resistive bearing system.
 12. The system of claim 7further comprising a slip ring comprising a fixed portion and arotatable portion, wherein the rotatable portion of the slip ring iscoupled to the tether.
 13. The system of claim 7 further comprising: aground station; and a slip ring comprising a fixed portion and arotatable portion, wherein the fixed portion of the slip ring is coupledto the ground station.
 14. The system of claim 7 further comprising aslip ring comprising a fixed portion and a rotatable portion, whereinthe fixed portion of the slip ring is coupled to the tether gimbalassembly.
 15. A system comprising: a tether comprising: a distal tetherend coupled to an aerial vehicle; a proximate tether end; and at leastone insulated electrical conductor coupled to the aerial vehicle,wherein the tether has an amount of twist between the distal tether endand the proximate tether end; a tether gimbal assembly, wherein thetether gimbal assembly is coupled to the tether and is rotatable aboutat least a horizontal axis or an azimuth axis; and a resistive bearingsystem coupled to the tether gimbal assembly, wherein the resistivebearing system is configured to allow the proximate tether end to rotateand to provide a resistance to the rotational torque of the tether so asto maintain the amount of twist in the tether within a determined rangeof values.
 16. The system of claim 15, wherein the resistive bearingsystem comprises a brake.
 17. The system of claim 15, wherein theresistive bearing system comprises a friction brake.
 18. The system ofclaim 15 further comprising a slip ring comprising a fixed portion and arotatable portion, wherein the rotatable portion of the slip ring iscoupled to the tether.
 19. The system of claim 15 further comprising: aground station; and a slip ring comprising a fixed portion and arotatable portion, wherein the fixed portion of the slip ring is coupledto the ground station.
 20. The system of claim 15 further comprising aslip ring comprising a fixed portion and a rotatable portion, whereinthe fixed portion of the slip ring is coupled to the tether gimbalassembly.